The study of aerobic energy metabolism in parasites and mammals was continued with efforts devoted primarily to elucidating the sequence of electron transfer in the respiratory chain of Entamoeba histolytica. Unique biochemical properties of the amoebae are their absence of heme proteins, large content of non-heme iron and acid-labile sulfide, minute amounts of quinones, and absence of covalently bound flavins. Hydrogen peroxide is formed only when artificial electron carriers mediate electron transfer from substrates to molecular oxygen, indicating the iron-sulfur centers and not flavin is the final electron carrier. Biochemical studies of Giardia lamblia were initiated this year. Respiratory metabolism of G. lamblia resembles that of E. histolytica by its lack of a functional tricarboxylic acid cycle and cytochromes; mediating electron transfer by flavoproteins and non-heme iron proteins. Only a small amount of acid-labile sulfide was detected in G. lamblia. Differences in the locus of the respiratory enzymes and substrate specificities are clearly evident in the aerobic metabolism of these two enteric protozoa. Another study initiated this year is the examination of respiratory metabolism of kinetoplasts from Leishmania donovani. Respiration was supported by Kreb's cycle substrates, particularly succinate, and the presence of functional NADH oxidoreductose was demonstrated. Mammalian studies centered on the assessment of whether protein synthesis or respiration was first affected during cellular heme depletion in cultured murine erythroleukemia cells.